Liquid metal pump for nuclear reactors

ABSTRACT

A pump for use in pumping high temperature liquids at high pressures, particularly liquid metals used to cool nuclear reactors. It is of the type in which the rotor is submerged in a sump but is fed by an inlet duct which bypasses the sump. A chamber, kept full of fluid, surrounds the pump casing into which fluid is bled from the pump discharge and from which fluid is fed to the rotor bearings and thence to the sump. This equalizes pressures inside and outside the pump casing and reduces or eliminates the thermal shock to the bearings and sump tank.

United States Patent 1191 Allen et al.

14 1 Oct. 7, 1975 1 LIQUID METAL PUMP FOR NUCLEAR REACTORS [75] Inventors: Harvey G. Allen, Murrysville; James R. Maloney, Irwin, both of Pa.

[22] Filed: Dec. 11, 1974 [21] Appl. No.: 531,589

3,467,015 9/1969 Allen 415/219 C 3,476,488 11/1969 Chambert 415/219 c 3,859,008 1/1975 Wieser 415/219 c FOREIGN PATENTS OR APPLICATIONS 124,621 9/1931 Austria 41'5/201 378,906 8/1923 Germany 415/201 Primary Examiner-Henry F. Raduazo Attorney, Agent, or Firm-Dean E. Carlson; Robert M. Poteat [5 7] ABSTRACT A pump for use in pumping high temperature liquids at high pressures, particularly liquid metals used to cool nuclear reactors. It is of the type in which the rotor is submerged in a sump but is fed by an inlet duct which bypasses the sump. A chamber, kept full of fluid, surrounds the pump easing into which fluid is bled from the pump discharge and from which fluid is fed to the rotor bearings and thence to the sump. This equalizes pressures inside and outside the pump casing and reduces or eliminates the thermal shock to the bearings and sump tank.

US. Patent 0a. 7,1975

'AIIIIII II 4 II All! This invention was mad'e in -the*course of, or-under a contract withthe United states Ato'rnic-Energy Commissionw- The invention relates generall to pumps for high metals. x

, Certain types of nuclear reactors are cooled by liquid metalpeig, sodiumffThe c oolant'is'nor'r nally supplied by a centrifugal pump. Ohetype involves a rotor mounted withiri a sumpjbut fed by a system that bypasses the sump so'as to. reduce head losses du 'eto energy losses, induced vorti ces and turbulence which can easily occur within'the sump-fluidi Sucha pump is shown in us. Pat. N6.3,467gO35'figranted Sept. 16, 1969 to Harvey G. Allen. W H F In an operating reactor system, suchjpumps" are sub ject to high mechanicalj' sti' essfes ahdthe r'malj'sh ock. They should be suitable for. handlihg'liq'uid meta l at temperatures up to l200F and for thermal shocks to 250F/20 seconds. I

Thermal shocks of this magnitude present several problems. They may limit the thickness of the metal to a value that is not acceptable under pressurization and other mechanical loading. This is particularly the case when thermal shock is applied to one surface only. It poses a serious threat to pump enclosure safety and problems with component functional reliability.

A hydrostatic radial bearing is an integral part of the pump. Bearing exposure to thermal shock will cause relative thermal growth between the close running clearances. Therefore, thermal shock protection of the bearing becomes a major problem.

It is therefore an object of this invention to provide a structure which will minimize mechanical stress in parts which are necessarily subject to thermal shocks, eliminate thermal shock in parts necessarily subject to stress and eliminate thermal shock to the bearing.

SUMMARY OF THE INVENTION This invention is directed to a pump of the type described above and illustrated by US. Pat. No. 3,467,015. Novel features enable it to remain structurally sound under very high temperature levels 1200F) and severe thermal transients (250F/2O seconds). This is accomplished by providing a chamber surrounding the pump casing and proving for bypass flow of limited quantities of liquid so as to provide a system in which pressure differences are minimized across walls which are subject to thermal shock on one surface. By this arrangement thermal shock is minimized on other walls,

which are subject to high pressure differentials, and thermal shock to the bearing is minimized.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing shows diagrammatically a vertical section through a pump embodying the invention;

DETAILED DESCRIPTION The pump is located in a tank 1 having a bottom portion 3 which is closed except for the inlet and outlet connectors and which forms a sump which during operation contains a body of liquid 5 having an upper surface 7. All parts of the pump except the drive shaft are I temperature, high' pressure liquids, particularly liquid located belowthe level of s'urfacel Within the tank is 'a'chamber 9 abovefwhichfis a support cylinder 11. 2 The upper portion ofchamber 91is formed by a radial bearing housing 13 within whichis a sodium lubricated bearing 15. Y

The pump'impeller 17=is journaled in bearing 15 and is driven by hollow drive shaft 19, which-is driven from overhead,- as shown in U.S.J'Pat-.'No. 3,467,015. The impeller 17 is of the overhung type, i.e., it is free. from shaft obstruction in the impeller eye 21.

' Fluidenters through frustoconical reducer 23 which isinount'ed in the wall of tank 1. It then is conducted by feed conduit'25 to eye 21 of impeller 17. Conduit 25 is slightly spaced from reducer 23 to leave a gap-27. The internal diameter of conduit 25 is slightly reduced adjacent its inlet end, so there is a slight venturi effect at-gap-27:- 5- i Impeller 17 operates in a pump casing 29, the upper portion 31 of which is integral with bearing housing 13. Theouter portion 33 of the pump casing includes diffuser' pas'sages'35 and straightening vanes 37 which removeth'e tangential componentof the fluid flow and increase the fluid static pressure. The liquid then flows out through discharge adapter 39 and discharge conduit 41, which forms a continuation of chamber 9.

The arrangements for minimizing thermal shock and mechanical stress will now be described. Openings 43 bleed a small proportion of the pump outlet flow into chamber 9, which is filled with liquid, thus equalizing the pressure inside and outside pump casing 29 and discharge adapter 39. Openings 44 deliver this liquid to the sodium lubricated bearings 15. Here the flow di vides. A part flows downwardly through openings 45 in hollow shaft 47 and into impeller 17. The remainder flows upwardly into the main body of fluid 5. It overflows through openings 49 in cylinder 11. This bearing discharge and internal leakage enters gap 27, mixes with the feed, and re-enters the impeller.

The relationship of the invention to the thermal shock and mechanical stress will now be described.

The thermal shock arises from sudden changes in temperature of the fluid entering inlet nozzle 23, due to reactor startup and changes in operating conditions. The high temperatures (up to 1200F) make it necessary to maintain the mechanical stresses comparatively low. This imposes conflicting requirements, since thin sections are desirable to resist thermal shock and thick sections are desirable to resist mechanical stress. The fluid holes 43 tend to equalize the pressures inside and outside the pump casing 29. The latter, which is subject to high thermal shock, may therefore be made comparatively thin. The dilution resulting from the comparatively small flow (say I% of the pump discharge) into the large volume of fluid in chamber 9 and tank 1 greatly reduces or completely eliminates thermal shock to these parts, which may be made of thick material.

As mentioned above, it is highly desirable to minimize thermal shock to the sodium-lubricated bearing 15. The molten sodium is supplied through holes 44 from the interior of chamber 9. As has been mentioned earlier, the relatively small flow through openings 43 is diluted by the large volume of fluid in chamber 9, smoothing out changes in temperatures and preventing thermal shock to the bearings.

The parts which are subject to full thermal shock on one side only, e.g., the pump casing 29, are made threeeighths inch or less in thickness. This is practical because the bleeding of fluid" equalizes pressure on both sides. The tank 1 and the chamber 9', on the other hand are protected from thermal shock. This permits them to be made as thick as desired to contain the full discharge pressure of the pump, typically about 250 lb./sq/in., and other forces.-The tank 1,:inparticular, is a major structural part of the pump system and is typically 2 inches or more in thickness. Other parts are of intermediate thickness.

It will be apparent from the above description that this invention equalizes the pressures on opposite sides of walls subject to thermal shock, permitting them to be made relatively thin, substantially eliminates thermal shock to thick walls, and substantially eliminates thermal shock to the bearings.

We claim:

1. A pump for use in pumping high temperatureliquid under high pressures comprising a tank closedat its bottom to form a sump, a substantially closed chamber within said tank and spaced therefrom, a substantially closed casing within said chamber and spaced from it over most of its area, but merging with the upper portion of said chamber to form a bearing housing and merging with the lower portion of said chamber to form an outlet duct, a centrifugal pump rotor suspended within and discharging into said casing and having a bearing within said bearing housing, an inlet duct passing through the walls of said tank, said chamber, and said casing and communicating with said pump rotor, passageways through the wall of said casing and through said bearing housing, said bearing communicating with said sump, whereby a portion of the liquid discharged from said pump will flow into the space between said casing and said chamber, then into said bearing, and finally into said sump, thereby equalizing pressures inside and outside said casing and reducing thermal shock to said chamber, bearing and tank on changes in temperatures in said inlet duct.

2. A pump as defined in claim 1 and further comprising an opening providing restricted communication between said inlet duct and the interior of said tank outside said chamber.

3. A pump as defined in claim 2 wherein said inlet duct comprises a frustoconical reducer passing through the wall of said tank and a feed conduit slightly spaced from the smaller end of said reducer and leading to said pump rotor. 

1. A pump for use in pumping high temperature liquid under high pressures comprising a tank closed at its bottom to form a sump, a substantially closed chamber within said tank and spaced therefrom, a substantially closed casing within said chamber and spaced from it over most of its area, but merging with the upper portion of said chamber to form a bearing housing and merging with the lower portion of said chamber to form an outlet duct, a centrifugal pump rotor suspended within and discharging into said casing and having a bearing within said bearing housing, an inlet duct passing through the walls of said tank, said chamber, and said casing and communicating with said pump rotor, passageways through the wall of said casing and through said bearing housing, said bearing communicating with said sump, whereby a portion of the liquid discharged from said pump will flow into the space between said casing and said chamber, then into said bearing, and finally into said sump, thereby equalizing pressures inside and outside said casing and reducing thermal shock to said chamber, bearing and tank on changes in temperatures in said inlet duct.
 2. A pump as defined in claim 1 and further comprising an opening providing restricted communication between said inlet duct and the interior of said tank outside said chamber.
 3. A pump as defined in claim 2 wherein said inlet duct comprises a frustoconical reducer passing through the wall of said tank and a feed conduit slightly spaced from the smaller end of said reducer and leading to said pump rotor. 